The present invention relates to a method of producing an optical element forming die capable of molding an optical surface of an optical element, and in particular, to a method of producing an optical element forming die, the method being capable of connecting a plurality of members accurately when forming a die with the plural members.
In the field of an optical pickup device which has been developed rapidly in recent years, an optical element such as an objective lens having a super precision form is used. Further, an optical element having a microscopic pattern such as a diffraction grating has been developed. When an optical element like that is formed of a material such as plastic or glass by using a die, products in a uniform shape can be manufactured rapidly. Molding with a die is thus suitable for mass production.
Incidentally, in many cases, a die for molding an optical element is manufactured in a way that the die is cut one by one with, for example, a single crystal diamond tool. Under this condition, it is necessary to make a die to be small for the purpose of keeping the cost to be low, because a material of a die is expensive. Further, when machining a die while the die is rotating after being chucked on a cylindrical grinding machine, for example, it is preferable that a die is light in weight and small in size, for inhibiting machining errors caused by centrifugal whirling of the die. However, if a die is small, the die needs to be connected solidly with a supporting member for transmitting properly the pressing force for molding, when mounting the die on a molding apparatus. Under the aforementioned condition, when a microscopic pattern is formed on a die transfer surface, such microscopic pattern needs to be positioned accurately for the supporting member, on a micron order basis.
In this case, when connecting the die with the supporting member, there are considered some methods of connection including connection by means of bolts, bonding by means of an adhesive agent, thermal spraying and connection by means of a low-melting point alloy. However, when the die is connected with the supporting member through the connection by means of bolts, there is a fear that bolts are loosened by the temperature cycle resulting from molding or by repeating force during its service for a long time. On the other hand, when the die is connected with the supporting member by means of an adhesive agent, there is a fear that the sufficient adhesion strength cannot be kept under the high temperature ambient in the course of molding. Further, when the die is connected with the supporting member by means of thermal spraying, there is a fear that the die or the supporting member is deformed by the temperature in the course of thermal spraying. Furthermore, when the die is connected with the supporting member by melting the low-melting point alloy and by making it to solidify, there is a fear that the die or the supporting member is deformed by being exposed to melted alloy at high temperature, when the melting point is high, and sufficient adhesion strength of the alloy cannot be secured under the high temperature ambient in the course of molding, when the melting point is low. When the microscopic pattern stated above is formed on the die, in particular, the methods of connection mentioned above cause serious problems. When the optical element is formed finally, the optical functions by the microscopic pattern are seriously affected.
Although Japanese TOKKAI No. 2001-217099 discloses a method to produce a cavity for cyclotron by connecting two members through electroforming, there is no description about a method of positioning the two members before the electroforming.